What adjective would you use to describe an idea that is capable of being successful?

The adjective that describes an idea as capable of being successful is "viable." This term signifies that something is practical and has the potential to be executed successfully. In the context of ideas, a viable idea is one that can be developed and stands a good chance of achieving its intended goals.

In contrast, "volatile" refers to something that is likely to change rapidly and unpredictably, often in a context that implies instability. "Vivacious" describes someone who is lively and spirited, which does not pertain to the effectiveness of an idea. "Vilify" means to speak or write about someone in an abusively disparaging manner, which is unrelated to the concept of an idea's success. Therefore, "viable" is the most appropriate choice for describing an idea with the potential for success.